The Magnetoresistance of Calcium-Doped Manganite Film and its Biepitaxial Step Junctions
|關鍵字:||磁電阻;雙軸階梯接面;相分離;鑭鈣錳氧;錳氧化物;晶界;自旋相關散射;magnetoresistance;biepitaxial step junction;phase separation;LCMO;manganites;grain boundary;spin-polarized scattering|
The extensive exploration for the magnetoresistance of the grain-boundary junctions and the properties of phase separation in calcium-doped manganite films are presented in this dissertation, respectively. In particular, the planar structure in a form of the biepitaxial step junction was adopted to investigate various behaviors of the grain-boundary effects. With the aid of scanning tunneling microscopy, the spatial variations of the local electronic structures in nanometer scale can be examined as well. Firstly, the biepitaxial La(0.7)Ca(0.3)MnO(3) thin films were grown on SrTiO(3) substrates using a buffer layer of anatase TiO2 and fabricated as a form of the planar structure of the biepitaxial step junction. The magnetoresistance of the biepitaxial step junction across the boundary layer of the biepitaxial (001)- and (110)-oriented films was investigated. The temperature and field dependence of magnetoresistance for the biepitaxial step junction are qualitatively similar to those of other types of artificial grain-boundary junctions with a comparable magnetoresistance ratio at low temperatures. However, the observed linear current-voltage characteristics across the biepitaxial step junction are in contrast to the commonly reported non-ohmic characteristics. Based on these experimental results, the biepitaxial step junction can be described by the model of spin-dependent transport across a depressed magnetic ordering and metallic-like junction layer. However, an intricate maze of the temperature dependent magnetoresistance behaviors is still unsolved in most types of grain-boundary structure. Furthermore, the attempts to explore the features of the grain-boundary effects in granular samples have been fulfilled by scanning tunneling microscopy in nanometer scale. We have confirmed the fact of phase separation in La(2/3)Ca(1/3)MnO(3) films. It is also worthy noting that the domains of the coexisting metallic-like and insulating-like phases is susceptible to the external magnetic field at the temperatures not far below the insulator-metal transition. The domain distributions vary in a irreversible manner. The observed irreversibility in the domain distribution suggests that the metallic percolation paths could be affected by the magnetic field. However, the magnetic field becomes ineffective on the domain distributions at lower temperatures. In addition, the correlation between the grain structure and the spatial distribution of the coexisting phases was evidently established. Due to the interplay between magnetic and electronic properties in manganites, the spin-polarized transport and the nanometer scale phase separation in manganites could be important to the understanding of the grain-boundary magnetoresistance. The reasons will be described in the contents of dissertation.